What does it mean if you're not sure whether you're pregnant but you start to bleed for no reason and your period isn't due for another 2-3 weeks?

If your period is due in 2-3 weeks and you are now bleeding, it could be implantation bleeding (when the fertilized egg implants into the uterine lining). This doesn't happen to everyone, but it would happen about 2 weeks before your period is due. This is usually just spotting and not heavy like a period. It also should not last long. You'll have to wait about 2 weeks to take a urine preg. test or a doctor could do a blood test in a few days to see if you're preg.